If the Mac setup assistant says that your user account already exists

Create a different user account that you can delete after completing setup.

When you set up a new Mac, a setup assistant opens and asks for setup details, including the name and password of your OS X user account. If you enter those details without completing the remaining setup steps, the setup assistant creates your OS X user account, but then opens again the next time you start up your Mac. If you enter the same account details again, it warns you that the account already exists. To resolve the issue:

Enter a different account name in the setup assistant, then complete the remaining setup steps.

When the setup assistant finishes setting up your Mac, choose Log Out from the Apple menu, then log in to your original account instead of the other account that you just created.